Trochoidal Milling Feeds and Speeds

I had a customer ask me how to calculate Feeds and Speeds for Trochoidal Milling with G-Wizard Calculator the other day. G-Wizard is a perfect tool for finding these Feeds and Speeds, but first, what is Trochoidal Milling?

Trochoidal Milling is a High Speed Machining (HSM) technique that moves the tool in a shape called a “Trochoid.” The link shows the derivation of what a Trochoid is, but here’s a typical Trochoidal Toolpath:

Using Trochoidal Milling to cut a slot…

The idea is each cut is a circular arc rather than moving down the slot in a straight line. The advantage of Trochoidal Milling is it keeps a constant load on the cutter so you can run higher feeds and speeds. Some Trochoidal paths create a more “D” shaped toolpath and don’t do a complete circle each time.

Now how do we use G-Wizard Calculator to figure out the Feeds and Speeds for such a path?

Actually, it’s pretty simple. Here is one all set up:

Feeds and Speeds for a Trochoidal Milling operation to cut a slot…

First, you set up the material and tool. We’re cutting some mild steel with a 1/2″ 4 flute carbide endmill.

Next, you need to tell it the cutting conditions. We’re doing a 1/2″ deep slot, so I went with a full 1/2″ of cut depth. Our cut width, or stepover is important because it governs how much we can accelerate due to High Speed Machining. The less of the tool that’s in the cut, due to smaller stepover, the faster we can go. But there’s a tradeoff. If you make the stepover too small, the cutter goes spinning through the material crazy fast but it doesn’t accomplish all that much and the MRR falls. A good starting point is a stepover of about 15% of tool diameter. You can fiddle with it from there, keeping an eye on the MRR, until you find a value that suits your needs.

Okay, the last step is to turn on the HSM Feeds and Speeds feature of G-Wizard. GW needs to know the Tool Engagement Angle to calculate HSM feeds and speeds, but it can figure that out from your stepover. Just simply click the “Const TEA” button and the Tool Engagement Angle is filled in. We can see for this example, it is 45 degrees. Say you’re doing a full slot, first pass, and the cutter is cutting full width because it isn’t using a Trochoidal path. That cutter has a TEA of 180 degrees and you’ll have to go much slower indeed.

Lastly, click the “Use HSM Feeds and Speeds” checkbox and your Feeds and Speeds are updated. We can see an RPM Factor and a Feedrate Factor that are each a little over 2. That means that since we can guarantee this is an HSM tool path that will keep a constant TEA, both the RPM and the Feedrate can be bumped up to over 2 times the conventional feedrates.

One thing you have to watch out for when using HSM toolpaths is your machine’s acceleration limits. Since you’re making continuous tight curves, if you make them too tight and/or bump up your feed rate too high, you can lose precision, even if your machine/tool could otherwise handle the feed rate.
This is something you sort of have to test for, because I don’t think it’s a specification that is easily found.

Steven, it’s a good point. I wouldn’t worry so much about accuracy unless you’re very far off on the acceleration requirements though. You only want to use HSM for roughing. Providing a large enough finish allowance will ensure the inaccuracy gets cleaned up on the finish pass.